The invention relates to a motor vehicle having a tank for storing a liquid reducing agent which can be supplied to the exhaust system of an internal-combustion engine provided in the motor vehicle, as well as having an air delivery device, by which an excess pressure can be built-up in an air cushion situated in the tank above the reducing agent level by supplying additional air. Concerning the technical background, reference is particularly made to German Patent document DE 10 2007 026 944 A1.
The injection of a liquid urea solution into the exhaust system of an internal-combustion engine upstream of a so-called reduction catalyst in order to be able to successfully convert harmful constituents of the internal-combustion engine exhaust gas flow in this reduction catalyst is basically known to a person skilled in the art. For an internal-combustion engine situated in motor vehicles, this urea solution representing a liquid reducing agent has to be carried along on board the vehicle. In this case, it is known that a problem to be solved consists of the fact that this liquid reducing agent may freeze at correspondingly low ambient temperatures and can then no longer be supplied to the exhaust system.
A further problem consists of the manner in which this liquid reducing agent is introduced into the exhaust system so that it may be distributed in the exhaust gas flow of the internal combustion engine in the best possible fashion. For this purpose, an injection together with a compressed-air flow has already been suggested. Here, it is, for example, also known from the above-mentioned German Patent document DE 10 2007 026 944 A1 to use the compressed air provided for representing this compressed-air flow also for the delivery of the liquid reducing agent to an injection nozzle provided in the exhaust system. In this case, by use of an air delivery device, an excess pressure can be built up for this compressed air in the cushion of air situated in the tank above the reducing agent level by supplying additional air. Under the influence of this excess pressure, reducing agent from the tank is then transported to the above-mentioned injection nozzle.
Furthermore, it is known that, for various reasons, two or more tanks may be provided for storing the liquid reducing agent in the motor vehicle. For example, merely because of insufficient space in the motor vehicle, it may be necessary to distribute a tank volume required for various reasons to two mutually separate tanks. It may therefore be necessary to provide, in addition to a first tank, from which the reducing agent is fed into the exhaust system, an additional so-called storage tank, from which liquid reducing agent is transferred into the above-mentioned first tank either continuously or as a function of at least one appropriately determined level (in the first tank).
As a function of the arrangement of the two or more tanks relative to one another and as a function of the course of a so-called supply duct, through which additional reducing agent from a further storage tank is supplied to the (first) tank, from which the conveying of reducing agent into the exhaust system takes place, it may be necessary to cause in a targeted manner the transport of further reducing agent through the supply duct by using energy or at least initiate such a transport. It is known to provide an independent pump as the delivery device for the liquid reducing agent for this purpose. However, such an independent pump represents an expenditure whose avoidance is the object of the present invention.
For a motor vehicle having a tank for storing a liquid reducing agent which can be supplied to the exhaust system of an internal-combustion engine provided in the motor vehicle, as well as having an air delivery device, by which an excess pressure can be built-up in an air cushion formed in the tank above the reducing agent level by supplying additional air, this object is achieved in that, by use of the air delivery device already provided (for generating an excess pressure in the cushion of air above the reducing agent level in the tank), as an alternative, a vacuum can be generated in the (above-mentioned) cushion of air, which has at least a supporting effect when reducing agent is fed into the tank. Preferably, the reducing agent can be supplied to the tank by way of this vacuum.
According to the invention, only a single delivery device for air is required, which is connected with the cushion of air in the tank above the reducing agent level. By use of this single air delivery device, liquid reducing agent can be removed from the tank, specifically in that an excess pressure is generated in the above-mentioned cushion of air with respect to an area into which the reducing agent is to be transferred, as well as can be supplied to the above-mentioned tank, specifically in that a vacuum is generated in the above-mentioned cushion of air with respect to an area from which further reducing agent is to be supplied to the tank. By creating such a vacuum, additional reducing agent is quasi-sucked into the tank, for example (or preferably) from an above-mentioned storage tank. As a function of the practical embodiment of the above-mentioned air delivery device, for generating an excess pressure or a vacuum in the above-mentioned cushion of air, for example, simply the rotating direction of the air delivery device can be reversed if, for example, the air delivery device is a simple vane pump. However, as an alternative, the generating of an excess pressure or a vacuum in the cushion of air in the above-mentioned tank above the reducing agent level is also possible by a suitable switching of valves which are provided in ducts appropriately connected with this air cushion or the surroundings as well as the air delivery device.
It is explicitly pointed out that, by use of the vacuum in the tank or in its cushion of air situated above the reducing agent level, not only reducing agent can be sucked into the tank from a storage tank provided in the vehicle but that preferably, after the vehicle is switched off, reducing agent can also be delivered back into the tank from a duct leading from the tank to the injection nozzle or the like in the exhaust pipe, particularly in order to avoid a freezing of the reducing agent in this duct. Such a vacuum may generally also be helpful when filling the tank with reducing agent; thus, also if no additional storage tank is provided in addition to this tank.
If, in addition to the above-mentioned first tank, an (additional) storage tank for liquid reducing agent is provided, this storage tank may be situated essentially at the same geodetic height as the first tank or even below the latter, which generally requires an energy supply for transferring liquid reducing agent from the storage tank into the first tank. In contrast, if the storage tank is geodetically situated above the first tank, a fluid can generally arrive in the first tank from the storage tank merely under the influence of gravity by way of a so-called supply duct. However, because of the risk that this supply duct may freeze, it may be desirable for fluid from the storage tank to arrive in the supply duct only on demand. For this purpose, the generally known principle of the siphon or pipette can now be utilized, according to which, by way of an at first geodetically ascending supply duct, fluid can be delivered from a storage tank into a geodetically lower-situated tank.
According to an advantageous further aspect of the present invention, by generating a vacuum in the cushion of air of the lower-situated tank, liquid reducing agent is therefore first sucked in from the storage tank by using the pipette principle and the hydrostatic pressure as well as the cohesion forces prevailing in the fluid column situated in the supply duct by way of the at first geodetically ascending supply duct, which reducing agent will then automatically flow from the storage tank over into the tank until a sufficiently high excess pressure is generated in the cushion of air of the tank, which pushes the above-mentioned fluid column in the supply duct back into the storage tank.
It was further recognized that, in connection with the removal of reducing agent from the tank, although excess pressure in the latter may be extremely useful under certain marginal conditions, it is not required continuously. This applies particularly if the injection nozzle, by way of which the reducing agent is injected into the exhaust system, is equipped with its own pump. It is basically known to provide a pump-nozzle unit as the injection nozzle, which sucks the required quantity of reducing agent out of the tank. However, at higher temperatures of the reducing agent and/or at a relatively low air pressure in the environment, a steam bubble formation may occur in the suction pipe of this pump-nozzle unit. As a remedial measure, an excess pressure should now be built-up in the air cushion in the above-mentioned tank above the reducing agent level. However, the air delivery device (for air from the environment) provided for this purpose, as an advantageous further development, should be operated only if this is actually necessary, i.e. if there is the danger of a steam bubble formation.
Accordingly, an independent delivery unit is therefore provided for the feeding of reducing agent into the exhaust system, which delivery unit sucks reducing agent out of the tank, in which case, by way of the air delivery device connected with the cushion of air in the tank, as a result of a suitable triggering of the air delivery unit, as a function of at least one suitable marginal condition, excess pressure is generated in the air cushion above the reducing agent level in the tank only to such an extent that no steam bubble formation can occur in the suction pipe of the above-mentioned delivery unit. A relevant temperature, preferably the temperature in the tank, and/or a relevant pressure, preferably the ambient air pressure, can be taken into account as such marginal conditions.
It was further recognized that a conclusion can be drawn concerning the filling level of reducing agent in the tank from the delivery output of the air delivery device for generating a certain excess pressure in the tank or in the cushion of air of the latter, which excess pressure can be determined by use of a sensor. This technique is generally known from the leak test of motor vehicle tank systems and can now also be used here at least for the plausibility of signals of an additional level sensor that is provided in the tank for the reducing agent level. Finally, with respect to a simple construction, the air delivery device can be connected by way of a venting duct of the tank with the cushion of air present in this tank, a venting of the tank taking place through the shut-down air delivery device.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawing.